Container for liquefied gas



Jam 1, 1957 F. J. ZIMMERMANN CONTAINER FOR LIQUEF'IED GAS Filed June 30. 1955 H547' TRANS/:EQ 1/5, Gaap/M575@ ,025550,05

United States Patent O M CONTAINER FOR LIQUEFIED GAS Francis J. Zimmermann, West Roxbury, Mass., assignor to Arthurl D. Little, Inc., Cambridge, Mass., a corporation of Massachusetts Application June 30, 1955, Serial No. 519,075

6 Claims. (Cl. 220-9) This invention relates to dewar-type containers and more particularly to improved surfaces for the lining of the vacuum chamber of the dewar-type containers.

In recent years many industrial processes and research projects have come to be carried out at temperatures at least as low as that attained by the use of liquid nitrogen (below -l95.8 C. or 77.3 K.). The handling, storing and transporting of the low temperature liquids required in such processes and research must be done by means of large (usually 25-liter capacity) dewar-type flasks which contain an inside vessel which has a complete double wall with the area between being evacuated. This vacuum chamber is the primary means of insulating the inside vessel and the liquefied gas or gases which it contains from the outside ambient temperature. The emissivity and heat transfer properties of the surfaces of these walls which face inward into the evacuated area of this vacuum chamber are of primary importance in designing an efficient dewar-type flask. This is due to the fact that often the major source of heat leakage in such a container is due to radiation heat transfer.

Although the description of this invention will be given hereinafter in terms of a dewar-type container or flask suitable for handling liquefied gases or liquids at temperatures of the order of liquid nitrogen or even liquid helium, the principle of depositing a finish showing extremely low emissivity and heat transfer is equally applicable to construction of other vessels, tank cars and apparatus used for handling extremely cold liquids by insulating them by a surrounding evacuated area.

Heretofore it has been customary to coat the facing inside walls of the vacuum chamber or jacket with a silver solution, if glass apparatus were used, to reduce emissivity and radiant heat transfer. Although this did materially cut down the loss of liquid gases due to vaporization in glass dewars, it was desirable to find an efficient means of treating these inside walls in the case of metal dewars. By the process of this invention it is possible now to reduce heat transfer across the Vacuum jacket to not more than one-half that shown by previous methods of surface treatment.

It is therefore an object of this invention to provide the inside walls of a vacuum jacket or those enclosing an evacuated area with surfaces which have emissivities markedly lower than heretofore achieved. It is another l object to provide such walls with surfaces which show markedly lower radiant heat transfer than previously achieved. It is a further object of this invention to provide more efficient dewar-type flasks, storage vessels and transfer lines for handling liquefied gases, or liquids which must be kept at very low temperatures.

The means for achieving this invention will be shown by the following discussion and with reference to the accompanying drawings in which Fig. 1 is a cross-section of a typical dewar-type flask or container embodying this invention; and

Fig. 2 is a plot of heat transfer as related to calorimeter pressure for two surface treatments.

Surfaces of extremely low emissivity are achieved in the process of this invention by electroplating the surface by means of a silver plating process. The surfaces are given a very brilliant luster and are not further treated by bung or other form of rubbing or polishing.

Fig. 1 shows a cross-section of a typical dewar-type container for liquefied gases such as air, nitrogen, oxygen, or hydrogen. The vacuum jacketed vessel with outer wall 12 and inner wall 16 and containing the liquefied gas 14, is suspended inside a metal container 10 and held in place by a neck ring 17. The inside area 13 between walls 15 and 16 of the vessel 15 is evacuated, and the inside surfaces 11 and 12 are treated to reduce their emissivity. The improvements in efficiency in a dewar-type container are achieved by the process of this invention by treating these surfaces 11 and 12.

The treatment of surfaces 11 and 12 consists of depositing a silver finish by electroplating from a bright plating bath to give an essentially mirror-bright coating, i. e., a coating having a reflectivity substantially the same as that of a mirror. A potassium cyanide plating bath containing organic addition agents such as for example thiourea, to contribute a positive brightening effect and to reduce grain size has been found satisfactory for depositing the necessary brilliant silver coating. Variables such as composition, current density and agitation may be determined experimentally to give the maximum brilliance to the final surface.

It is essential that no bufiing step be used in lobtaining the brilliant surface of this invention. Extensive tests have shown that when electroplated surfaces are buffed their emissivities and their ability to transfer radiant heat are markedly increased, thus decreasing their efficiencies for such uses as considered here. Although the reason for this is not precisely known, it is known that buffing does change a surfaces physical properties such as its electrical resistance.

The inner and outer shells may be of any suitable metal such as stainless steel or copper or of a rigid plastic material coated with a conducting film suitable to serve as an electrode in the plating bath.

Although the thickness of the silver finish formed on these surfaces does not appear to be critical, it is desirable to maintain it at a minimum from an economic point of View. Thicknesses of approximately 0.0003 inch have been found to be very satisfactory.

The vacuum jacket of a liter liquid nitrogen dewar, such as is illustrated in Fig. 1`, was used' as a modified' calorimeter to measure the relative effectiveness of var-ious surfaces. The calorimeter was so set up and instrumented that the amount of vacuum within the evacuated area, corresponding to area 13 in Fig. 1, could be varied, the quantity of liquid boiled off could be measured and the amount of radiant heat transferred from surface 12 to surface 1l could be measured.

Heretofore, gold-plated finishes had been considered effective and efficient surfaces for metal vacuum-jacketed vessels in dewar-type containers. In Fig. 2 there are plotted data showing the variation of heat transfer with pressure in the evacuated area. The heat transferred at zero pressure is essentially diie to radiation in this apparatus. It will be `seen from Fig. 2 that inthis test the bright silver finish of this invention transfers by radiation just about one-half the amount ofV radiant heat transferred by the gold plated 'surfaces when the data lines are extrapolated to zero mm. Hg.

Further testing with the calorimeter was carried out by coa-ting the surfaces involved'in various ways and measuring the emissivity factors. These factors were obtained by measuring the amount of boil-offV liquid nitrogen when the outside ofthe vacuum jacket was at room temperature. From these data the amount of heat transferred across the vacuum jacket (and hence by the treated surfaces) could be calculated; and from this the emissivity factor which is in a direct ratio to the heat transferred by radiation in the equipment tofthe heat that would be transferred if both surfaces were black-bodies could be determined.

The resulting data are shown in the table in which there is indicated 4the relative merit of various combinations of surfaces. All of the surfaces except the polished copper and stainless steel were electroplated. Most of the tests combined a polished gold-platedwshell with spheres of various treatments in order that an indication of the relative emissivities of the nitrogen cooled surface (represented by surface 11 of Fig. l) would be shown directly by the emissivity factor.

It is readily seen that two silver surfaces prepared in accordance with this invention lare the best combination, having a heat leakage approximately` one-half that of two polished gold surfaces. However, `if only the surface facing theevacuated area of the wall which comes in contact with the liquid to be stored or handled is coated to give a finish as described by this invention, the emissivity factor Iis reduced. to 0.0061 (weighted average) if the surface of the outer wall is finished in polished gold. This-should be comparedwith' 0.0103 (weighted average) for both surfaces lbeing iinishedin polished gold.

It will be seen from the`examples`randdata shown in Table I that the brilliant unbuffedsilver finish* for both" the inside surfaces -of the'vacuum jacket shows a markedV increase in efficiency over any of theiinishes used heretofore, including polished gold. Such increased efiiciency' is shown yby the fact'that heattransfer, andalso emissivity, is reduced by the brililantsilver surfaces of this invention to about one-half that shown by the best previous surfaces. Thus, by using the'surfacefinishes of this invent-ion it is possible to make" dewar-type containers which can store or transport liquefied `gases with far lessV tus, sueltas transfer lines, which'are required to insulate liquefied gases' or other liquids'whichinust be 'handledat very low temperatures.

4 TABLE 1 Ernssvty factors for various combinations of the inner surfaces of an evacuated vessel Shell Surface (Room Iemperature)LL Sphere Surface (Liquid N2 Temperature)b Brilliant; Unbuifed Silver Polished Polished Matte G Copper old Silver Brilliant unbuiied silver.

Matte silver Dry buffed silver. Matte gold Gold Wash Dry bufed go1d Polished gold Polished copper Stainless steel spinningrubbed with fine emery cloth Drafting tape Coi-responds to surface ri Fig. 1. b Corresponds to surface 1l of Fig. 1. The term matte is used to describe a finish Which 1s somewhat cloudy or milky in appearance.

d Stainless steel sphere-all'other copper sphere.

jacketed inner vessel for handling liquefied gases, the

wall's'-of said vessel facing the evacuated area having a brilliant unbuffed silver finish, said silver finish-having beendepositedby electroplating'from a potassium cyanide' bath containingorga'nic additiveswhich reduce the'particle size of` the silver in said silver finish and which contribute a'positive brightening effect.

3; A' dewar-type container in` accordance with4 claim 2 wherein vsaidwalls are stainless steel.

4. A dewar-type container in accordance with claim 2 wherein 'said walls-are copper. l

5.' A dewar-type container in accordance withclaim'2 wherein said walls'are a'rigid plasticmateriall covered with'a conducting coating.

6. A dewar-type container equipped with a vacuumjacketedinner vessel for handling liquefied gases, Vthe wall of-said inner vesselswhich'is in contact withsaidliquefiedI gasesA having its surfacefacing the evacuated-area coated witha: brilliant unbuffed silver finish, said' finish havingV been deposited by electroplating from a potassium cyanide bath'containing'organic additives whichreduce the particle size of the silver insaid silver finish and which'contribute a positive brightening effect.

ReferencesY Cited inthe file of this patent UNITED STATES PATENTS 1,816,476 Fink etal; luly 28, 1931 2,396,459 Dana-et al. Mar. 12, 1946 2,643,021 Freedman" June 23, 1953 FOREIGN PATENTS 483,207 Great'Britain Apr. 13,V 1938 OTHER REFERENCES E'gberg et al.:A Article Brighteners in Silver Plating Solutions, printedby The Electrochemical Society, preprint`74J-13; released' October 17, 1938,' pages 195,H 197- 199. 

1. WALLS ENCLOSING AN EVACUATED AREA, THE SURFACES OF SAID WALLS FACING EACH OTHER HAVING A BRILLIANT UNBUFFED SILVER FINISH, SAID SILVER FINISH HAVING BEEN DEPOSITED BY ELECTROPLATING FROM A POTASSIUM CYANIDE BATH CONTAINING ORGANIC ADDITIVES WHICH REDUCE THE PARTICLE SIZE OF THE SILVER IN SAID SILVER FINISH AND WHICH CONTRINUTE A POSITIVE BRIGHTENING EFFECT. 